The diversity receivers found in the prior art first compare estimated received power values of the carrier waves of the received signals on each of two demodulation paths at each point in time, and select and output the received signal with the larger estimated value; this is generally known as selection diversity (also referred to below as the selection system or selective diversity). That is, of the two received signals at each point in time, they selectively output the received signal with the better reception conditions, and do not use the received signal with the inferior reception conditions. At each point in time, accordingly, they cannot obtain better receiving performance than the individual received power obtained from one of the received signals in the two demodulation paths.
To improve the receiving performance further, combining the two received signals has been contemplated.
A type of diversity receiver is known that employs a maximal ratio combining diversity system by providing circuitry that calculates a ratio of power levels (estimated power values) of a pair of received signals on a pair of demodulation paths (or demodulated signals obtained by demodulating the received signals), generates weighting coefficients according to the calculated power ratio, and multiplies the received signals by the weighting coefficients to create a weighted combination.
It is known, as shown in “Improvement of terrestrial digital TV broadcasting performance by diversity receiving” by Takashi Seki, et al., Technology Report from Image Information Media Academy, May 25, 2001, Vol. 25, No. 34, pp. 1 to 6, ROFT2001-54 (May, 2001), that a maximal ratio combining diversity receiver can not only mitigate multipath distortion, as do diversity receivers using the selection diversity system, but also improve transmission characteristics with respect to thermal noise, and can further improve the instantaneous carrier-to-noise ratio (also referred to simply as the CNR below).
Equal gain combining diversity receivers are another example of a diversity system in which a pair of received signals on a pair of demodulation paths are combined to improve receiving performance. Equal gain combining diversity always combines a pair of received signals with equal gain, so that regardless of the power levels (estimated power values) of the received signals on the pair of demodulation paths, the average value of the received signals on the demodulation paths is always output as the combined signal. It is known that equal gain combining diversity produces a larger diversity effect than selection diversity and a smaller diversity effect than maximal ratio combining diversity. By contrast, when the difference between the received signals on the pair of demodulation paths (or the demodulated signals obtained by demodulation of the received signals) or between the CNRs of the received signals increases, the receiving performance of equal gain combining diversity may fall below that of selection diversity.
Among conventional diversity receivers, selection diversity receivers, for example, can operate with small circuitry because they simply use one of the received signals on the pair of demodulation paths, but there has been a problem in that it is difficult to improve their receiving performance.
Although equal gain combining diversity receivers require only simple equalizers to be added and can accordingly operate with comparatively small circuitry, and although they can provide better reception than with selection diversity, there has been a problem in that their reception cannot be improved over that of maximal ratio combining diversity. There has also been a problem in that as the difference between the received signals on the pair of demodulation paths increases, the receiving performance of equal gain combining diversity receivers is degraded.
Maximal ratio combining diversity receivers can provide better receiving performance than selection or equal gain combining diversity receivers, but there has been a problem in that they require circuitry for generating weighting coefficients according to the (estimated) received signal power ratio and further multiplying the received signal powers by the weighting coefficients, resulting in larger circuit scale.
The present invention is intended to solve problems such as those above, and has the object of providing a diversity receiver with a small circuit scale in which the receiving performance can be improved to a level near that of a maximal ratio combining diversity receiver.